It is known to propel a boat or other watercraft using a water jet apparatus mounted to the hull, with the powerhead being placed inside (inboard) or outside (outboard) the hull. The drive shaft of the water jet apparatus is coupled to the output shaft of the motor. The impeller is mounted on the drive shaft and installed in a housing, the interior surface of which defines a water tunnel. The impeller is designed such that during motor operation, the rotating impeller impels water rearward through the water tunnel. The reaction force propels the boat forward.
The overall performance of a water jet propulsion system is highly dependent on the size, i.e., area, of the exit nozzle outlet. A relatively large opening is required to provide initial thrust, whereas a relatively small opening is desirable for high speed. A smaller opening is also desirable for low-speed maneuvering, as it would result in a higher velocity of the exiting water flow at low engine rpm. Consequently, an adjustable exit nozzle outlet would be desirable to provide improved overall performance.
Some conventional adjustable exit nozzles for water jet propulsion systems have been designed using the iris method for opening or closing down the exit nozzle outlet, thereby increasing or decreasing the diameter of the outlet. While these means produce the desired results, an iris-type structure has a tendency to clog with silt or debris present in the water being pumped through the water jet propulsion system. This clogging eventually causes the adjustment mechanism to become inoperable.
Another technique for providing an adjustable exit nozzle outlet area is disclosed in U.S. Pat. No. 5,244,425 to Tasaki et al. An arrangement is provided for variably adjusting the effective flow area along a region within a water jet propulsion system. A bullet-shaped adjusting cone is moveable in and out of the outlet region, thereby adjusting the area of the outlet region. The adjusting cone is housed with a hub of the impeller assembly. A front portion of the cone member is flanged and forms an actuator. A spring is also housed within the hub, around a shaft connecting the cone portion and the actuator, and provides a forward biasing force against the actuator, thereby positioning the adjusting cone out of the outlet region and maximizing the outlet region's area. A rod and lever assembly is used to overcome the spring force, thereby moving the adjusting cone rearwardly and into the outlet region in order to reduce the area of that region.
Another type of variable nozzle employs a resilient or elastic member which can be expanded and contracted by the supply and withdrawal of a fluid from a chamber formed by the elastic member. For example, U.S. Pat. No. 3,279,704 to Englehart et al. discloses a water jet device which incorporates a control element which is moveable in and out of the outlet region and an annular flexible member made of rubber or like material which is installed in the outlet region. The flexible member and the outlet region form an internal hollow portion within which fluid pressure can be varied to adjust the position of the flexible member, thereby adjusting the thickness of the annular flow of water between the control element and the flexible member.
There is a need for a means by which the area of the exit nozzle outlet of a water jet propulsion system can be adjusted without the possibility of clogging, without use of a complicated linkage system and without redesigning the exit nozzle itself.